Structural anomalies at the magnetic transition in centrosymmetric ${\mathrm{BiMnO}}_3$

The structural properties of ${\mathrm{BiMnO}}_3$ were determined from neutron powder diffraction data as a function of temperature and magnetic field. The structure at all temperatures was found to be centrosymmetric with space group $C2∕c$, which is incompatible with ferroelectricity. At $T_C\simeq 100\mathrm{K}$, we observed the onset of a large magnetoelastic strain, proportional to the square of the magnetization. We interpret this structural rearrangement, together with the previously observed magnetodielectric anomalies, due to the need to optimize the partially frustrated magnetic interactions.

Figure 1

Main panel: Magnetization vs temperature for ${\mathrm{BiMnO}}_3$, collected on both heating and cooling in a $100\mathrm{Oe}$ magnetic field. The anomaly at $42\mathrm{K}$ (arrow) is due to a small amount of antiferromagnetic ${\mathrm{Bi}}_2{\mathrm{Mn}}_4{\mathrm{O}}_{10}$, which does not show hysteretic behavior. Inset: Mn magnetic moment vs temperature, as determined from neutron diffraction data (see text).

Figure 2

(Color online) Orbital ordering pattern in ${\mathrm{BiMnO}}_3$. Main panel: A slice cut perpendicular to the pseudocubic ${\mathrm{c}}_p$ axis, showing ${\mathrm{MnO}}_6$ octahedra with in-plane (red, dark gray in the printed figure) and out-of-plane (light blue, light grey in the printed figure) long $\mathrm{Mn}\text{−}\mathrm{O}$ bonds, corresponding to occupied $d_{z^2-r^2}$ $e_g$ orbitals. Inset: The same structure shown on the monoclinic axes. The presentation emphasizes the occupied orbitals.

Figure 3

Magnetoelastic effect in ${\mathrm{BiMnO}}_3$. Bottom: Relative change $\left[\mathrm{\Delta }p\right(T)∕p(300\mathrm{K}\left)\right]$ of the monoclinic lattice parameters $p=a$, $b$, and $c$. Middle: Change of the monoclinic $\beta$ angle vs temperature. The parameters were determined from Rietveld refinements based on neutron diffraction data (symbols). The lines represent the calculated thermal expansion due to phonons, based on a simple Einstein fit to the high-temperature data with $T_E=200\mathrm{K}$ (see text). Top: excess thermal evolution (i.e., with the phonon contribution subtracted off) of the pseudocubic lattice parameters. Lines are guides to the eye. If not shown, the error bars are smaller than the symbols.

Figure 4

Change of the monoclinic $\beta$ angle as a function of the square of the ordered magnetic moment on the Mn site, as obtained by varying the temperature in zero field (circles) and by varying the applied magnetic field in the range $0–10\mathrm{T}$ at a constant temperature of $95\mathrm{K}$ (squares).